This article provides a legal analysis of the largely uncharted notion of "acts harmful to the enemy" under international humanitarian law, which reconciles the humanitarian need to grant special protection to medical services (medical personnel, units and transports) in the interests of the wounded and sick with the military necessity to remove it when acts are committed contrary to good faith and for hostile purposes or with effects which harm the adverse party. The meaning of the notion is clarified by primarily looking into the legality of an attack against land-based medical services by the aggrieved party to the conflict as a consequence of harmful acts. It concludes with specific recommendations on how to interpret the law governing such an attack, considered prima facie lawful, on a hospital.